Pyongyang called the 11-day operation a "reckless" invasion rehearsal that could trigger an "uncontrollable phase of a nuclear war".

China and Russia last week urged the United States to suspend the drills in exchange for North Korea suspending its missile and nuclear tests.

Earlier this month, US President Donald Trump vowed to respond to North Korea's actions with "fire and fury".

North Korea, for its part, threatened to fire a salvo of missiles towards the US territory of Guam - a plan that its leader Kim Jong-un delayed last week.

Kim warned the plan could still go ahead depending on Washington's next move.

South Korean protesters hold placards that read 'stop war exercise' during a rally denouncing the military exercise [Jung Yeon-Je/AFP]

The number of US soldiers in this year's drill was reduced by a third, while media in South Korea reported that the US was considering scrapping a plan to bring two aircraft carriers to the peninsula.

But US Defense Secretary James Mattis said on Sunday that the smaller troop numbers were "by design to achieve the exercise objectives", denying suggestions that Washington had cut them back to try to ease tensions with Pyongyang.

Al Jazeera's Step Vaessen, reporting from Seoul, said, "the crucial aspect will be how North Korea responds".

In 2016, Pyongyang retaliated by firing a submarine launch missile, which ended up in the Sea of Japan.

"But this time around, given the recent tensions, many are saying the North will carefully calculate to these drills if the country is really serious about some room for negotiation," our correspondent said.

On the eve of the UFG drills, North Korea said the US was "pouring gasoline on fire".

In a commentary carried by the official Rodong Sinmun newspaper, North Korea said Washington was "mistaken" to think that a nuclear war would take place on "somebody else's doorstep far away from them across the Pacific".